Cooling System Troubleshooting

Revision D – 02/07/2009


The Miata cooling system is described on pages E-2 through E-14 of the 1990 Miata Factory Workshop Manual, and much of the material below, including illustrations, is taken from that manual. Model years beyond 1993 have a number of differences, but the basic troubleshooting procedures should be similar. Hopefully this article will be useful to many of those having cooling issues. However, there is really no substitute for the FWM if detailed information is needed.


A knowledge of what conditions should exist in the cooling system is invaluable in troubleshooting. So for the benefit of those who do not have a complete set of manuals for their car, this article starts with a somewhat comprehensive set of desired cooling system parameters.




Coolant Capacity and Mixture


The cooling system capacity is 6.3 U.S. quarts. Mazdas recommended anti-freeze solutions for the Miata are as follows:


Minimum Temperature

Volume Percentage (%)

Specific Gravity @ 68F



-16C (3F)




-26C (-15F)




-40C (-40F)




Table 1. Coolant Mixture Recommendations (Mazda)


Remember that the primary task of liquid cooling is to transfer heat energy from the metal parts of the engine to the radiator and then to the air. Water has a specific heat of 1.00, but the specific heat of glycol is only 0.571, meaning that a given quantity of glycol will carry away only about 57% of the heat that the same volume of water will transfer.


A more complete table of freezing points vs. % ethylene glycol is given in the Handbook of Chemistry and Physics:




Volume Percentage (%)

Specific Gravity

@ 60F



-3.9C (25F)




-6.7C (20F)




-12.2C (10F)




-17.8C (0F)




-23.3C (-10F)




-28.9C (-20F)




-34.4C (-30F)




-40C (-40F)




Table II. Freezing Point vs. % Ethylene Glycol


The boiling point of water at 14.7 psia is 212F, and if the pressure is increased, it rises. The pressure in the Miata cooling system should be between 11 and 15 psi (25.7 to 29.7 psia) maximum, which would place the boiling point of a 50/50 coolant mixture at around 265F (at 15 psi) according to the Prestone labels.


Unfortunately I could not find information correlating the H reading on the Miata temperature gauge with coolant temperature. The 1990 FWM is written as though the Water Thermosensor is the temperature gauge sensor, which it is not. There is no data on the actual gauge sensor, although the manual spec on the gauge itself states that the gauge should point to H for a resistance to chassis ground of 20 ohms (18 ohms for Canadian cars).


Coolant Flow


The coolant flow is shown in Figure 1. The arrows clearly show the flow, but the diameters of the heater hoses are actually considerably smaller than those for the radiator hoses, as you can tell from looking at your engine bay. If the thermostat is removed, flow will be unbalanced, since the water pump and the thermostat housing are both at the front of the head. As a result, the rear cylinders will run hotter than the front. The lesson here is to keep the thermostat installed; if you have cooling problems, removing the thermostat is not a good long-term solution.


Note that the diagram shows an IG relay. That is actually the Cooling Fan Relay, as you can see on the cover of the Main Relay and Fuse Box.

Figure 1. Coolant Flow

Note that there is no coolant shutoff valve at the heater hose connections. Coolant always flows through the heater core regardless of the position of the heater controls in the passenger compartment. It is very important that the heater core path from the rear of the head to the water pump inlet not be blocked, since otherwise hot coolant in the rear of the cylinder head will stagnate, making the rear of the head overheat.


Note the small hose at the bottom of the thermostat housing that carries coolant output from the cylinder head through the input side of the thermostat housing and down to the water pump input. This assures that hot coolant from the head flows through the thermostat housing when the thermostat is closed, exposing the thermostat valve to the coolant. If this path is blocked by metal shavings in the thermostat housing outlet nipple, or anywhere in the small hose, the coolant will not flow through, and the thermostat will not open early enough. A symptom of a blocked path here would be temperature rising above normal on first warm-up, then suddenly dropping to normal sometime later. This connection can be seen in the lower right portion of Figure 2.


Figure 2. Thermostat Housing


Some coolant flows from the cylinder head into the Idle Speed Control valve below the throttle body (called the Idle Air Control valve on the 1.8L engine), then out of the ISC valve and through the Air Valve mounted on the engine side of the intake manifold, returning to a tee into the down tube from the thermostat to the water pump input. (1.8L engines do not have an Air Valve.) The Air Valve on the 1.6L engine is open when the coolant is cold, raising the idle speed, and closes gradually as the coolant warms up to normal operating temperature.


Thermostat and Water Thermoswitch


The OEM thermostat opens in two stages. There is a sub-valve that opens at 182 – 188F, and a main valve that opens at 188 – 193F. As shown in Figure 2, the main valve is oriented to the front of the thermostat housing. On the cover of the thermostat housing is mounted a Water Thermoswitch. This switch is open when coolant temperature is below approximately 207F and closed when the temperature is higher. The closed switch activates the Radiator Cooling fan. If A/C is installed, but not turned on, the A/C Condenser Fan (on the right side of the radiator looking forward) will not activate when the switch is closed.


Fan(s) and Fan Relay


The base Miata has one fan, mounted on the left (looking forward) side of the radiator, called the Radiator Cooling Fan. The Radiator Cooling Fan motor should use 5.3 – 6.5 Amps. If A/C is installed, there is a second fan on the right side of the radiator called by Mazda the A/C Condenser Fan. The A/C fan motor is smaller and the fan shroud fits on differently spaced mounting points, so the two fans are not interchangeable.


As seen in Figure 3, when the Water Thermoswitch is closed, the Cooling Fan Relay contacts close and power the Radiator Cooling Fan. The connection to the Diagnosis Connector goes to its TFA terminal. If the TFA terminal is jumpered to the GND terminal, the effect is the same as closing the Water Thermoswitch, i.e. the fan should activate if the ignition switch is on. This is a good test to determine that the relay and fan motor are working. (This test will not activate the A/C Condenser Fan.)


If the test above fails to activate the Radiator Cooling fan, then the 30 amp Cooling Fan fuse in the Main Relay and Fuse Box is blown, or the Cooling Fan Relay or Main Relay has failed open, or the fan motor is bad.


Figure 3. Cooling Fan Wiring Diagram


A/C Condenser Fan Circuit


The A/C Condenser Fan is activated by the A/C on/off switch, on the heater/air conditioning control panel. When the A/C switch is on, both the A/C Condenser Fan and the Radiator Cooling Fan are activated by the ECU, through the A/C relay. Figure 4 shows the wiring for this circuit.


Figure 4. Air Conditioning Condenser Fan Circuit


Radiator Cap


The OEM caps positive pressure valve should hold closed until at least 11 psi and not more than 15 psi pressure. The negative pressure valve should be easily opened with two fingers and should seal closed when released.


Coolant Reservoir


This tank connects to the radiator at the cap, just above the positive pressure valve. The tank is not under radiator pressure. It should be sufficiently full to allow coolant to be sucked back into the radiator as it cools off. If air is sucked in instead of coolant, it will expand in the cooling system as the engine warms up, and will result in air and coolant being expelled back into the reservoir if the pressure is over 15 psi.


The small tube from the reservoir input to the bottom of the tank sometimes gets clogged with rust or other deposits from the cooling system. It should be cleaned for good operation of the system.


The other tube from the reservoir is an overflow to vent excess air and coolant. If the hoses are removed from the reservoir cap, be sure to reassemble with the caps yellow tab pointing toward the rear of the engine; the long tube connects to the radiator hose.


Bleeding Air From the System


Elevate the car on ramps or jack stands; this will ensure that the cooling system radiator cap is above the thermostat hosing.  Drain the cooling system into at least a 3 gallon bucket and flush the cooling system before installing any new components if possible. Flushing the system will ensure that most of the contaminants are removed from the cooling system. Most drain plugs are in the center of the radiator; some may not line up with the drain plug opening in the plastic under-shield. In that case, the shield has to be removed before draining.


Fill the radiator with the coolant mix until the level is about 1/2 to 3/4 inch above the core and then start the engine. Keep a bucket under the center of the car just in front of the air dam under the front of the car. Run the engine, keeping this fluid level by topping off occasionally. The engine will come up to operating temperature, and the Radiator Cooling fan will kick on.  


The system will overflow 2 to 3 times. Each overflow should occur as the radiator cooling fans kick on so make sure you have a bucket under the car. Cycle the system until it does not overflow again. After each surge and overflow, refill the radiator with the engine still running.


Turn the engine off and let it cool for about 1/2 hour. Then fill the radiator almost up to the water neck. Squeeze the upper radiator hose to bleed out the excess air, and then top it off again.  


Install the radiator cap and run the engine until the fans kick on again, then check for leaks.







The OEM radiator is aluminum with plastic end caps. With age, the black plastic will turn a brownish-green color and develop the appearance of hairline cracks. At this time leakage may begin to occur at the plastic/aluminum interface. Replacement of the radiator is recommended if this happens.


The water pump may also begin to leak, as evidenced by coolant on the lower front of the oil pan and/or engine block. Again this usually means the pump is due for replacement, although the pump gasket might be the culprit.


Any of the hoses shown in Figure 1 can deteriorate and begin leaking. If a heater hose splits, be very careful removing it from the heater interface tube, which is thin copper and will deform easily. If the hose has to be replaced anyway, its best to slit the end of the hose along its length where it slides over the tube, instead of trying to pull it off or pry it off. If the tube is deformed it may be prone to leaking after hose replacement.


There is a rubber cap over the end of an unused coolant tap at the rear of the engine block shown as cursed water plug in Figure 5 (Thanks for the picture, Harry Sue.)

This cap has been known to leak and is difficult to trace, since it cannot be seen from the top or bottom of the engine. To get at it with the engine in the bay, I think the CAS cover has to be removed. However, I have never had to do this and I may be wrong about that.


A hose leak at the rear coolant housing generally requires the coil assembly to be removed before trying to release the hose clamp.



Figure 5. Coolant Cap at Rear of Engine [Rev. D]





Mazda states in the 1990 FWM the following causes and remedies for overheating:


Possible Cause


Coolant level insufficient

Coolant leakage

Radiator fins clogged

Radiator cap malfunction

Cooling fan malfunction

Thermostat malfunction

Water passage clogged

Water pump malfunction










Maybe we can be more specific


Overheating at Idle


At idle, if the temperature gauge indicates above normal, the Radiator Cooling Fan should be running. If not, test with the TFA to GND jumper in the Diagnostic Connector and see whether the fan turns on when the ignition is on. If it does, the Water Thermoswitch needs to be replaced, its connector needs to be cleaned, or its wiring needs repair. If not, and there are no other problems with the engine that are electrical in nature, either the Cooling Fan Relay (the IG relay in Figure 1) or the fan motor is bad. Of course there is always the possibility of an unplugged connector somewhere, or a broken wire. In a few cases the fan blades had come loose from the motor shaft and were not turning, although the motor was OK.


If the fan is running but the engine still is overheating at idle, feel the top radiator hose. It should be HOT! If not, the thermostat may be stuck closed. If it is hot, feel the bottom radiator hose. It should be warm, but perhaps 20F cooler than the top hose. If it is cool, there is blockage in the radiator, or possibly in the lower radiator hoses, preventing flow into the water pump. Sometimes old bottom hoses collapse under a vacuum created in them by the water pump straining to pull in water at its input.


Finally, check to make sure there is no blockage in the heater hoses, which would stagnate the water at the rear of the head where the temp gauge sensor is located, and cause the temperature there to be much higher than at the front of the engine.


Overheating at Speed


This problem, assuming there is adequate uncontaminated coolant in the system, is either poor water flow through the Figure 1 path, or poor airflow through the radiator.


Poor water flow can be caused by a stuck thermostat (only partially open), collapsed hose, clogged radiator, or a bad water pump.


Poor airflow through the radiator can be caused by air blockage due to a license plate configuration, bugs etc. stuck in the fins, bent fins due to mishandling, and in rare cases, the Radiator Cooling Fan running backwards!


If all of the above items have been checked and the engine still overheats, the cooling system should be flushed to remove any scaling or corrosion inside the water passages.


Draining and refilling of the coolant with a lower percentage of glycol in the mixture (see Tables I and II) would help if the glycol percentage is too high (over 50%).


Also keep in mind that temperature sensors and gauges do go bad occasionally. If the gauge says the engine is too hot, but there are no other apparent symptoms like lower-than-normal oil pressure, steam or bubbles escaping into the coolant reservoir, or a very hot under hood temperature, maybe the gauge is giving a false reading.


Figure 6 is a page from the FWM showing radiator removal, and Figure 7 shows water pump removal. Figure 8 is a simplified block diagram of the electric cooling fan system.

Figure 9 is a troubleshooting tree for reference.


Figure 6. Radiator Removal


Figure 7. Water Pump Removal


Figure 8. Simplified Block Diagram – Cooling Fan Wiring